1. Field of the Invention
The present invention relates to a photoelectric conversion device, and more particularly a photoelectric conversion device adapted for use in an image reading apparatus such as a facsimile apparatus, an image reader, a digital copying machine, an electronic black-board or the like.
2. Related Background Art
In recent years there have been developed long line sensors having a same-size optical system, as the image conversion device for the facsimile apparatus, the image reader or the like. This is mainly due to the compactization and high density structure of the photoelectric conversion device enabled by the progress in the technology of printed-circuit, wiring boards. In the long image sensor having the same-size optical system, a linear array of photoelectric converting elements is capable of reading an original image in contact therewith, so that the apparatus can be compactized and can achieve a high image reading speed in comparison with the conventional method of reading the original image point by point with a photoelectric converting element and an optical system utilizing lenses
In the conventional long line sensor with same-size optical system, the photoelectric converting elements constituting an array are respectively connected to signal processing IC's composed, for example, of switch devices. However, for example in the G3 facsimile format, 1728 photoelectric converting elements are required for A4 size originals are required, therefore many signal processing IC's are required. For this reason, there are required a large number of device mounting steps, and there has not been obtained an apparatus satisfactory in terms of production cost and reliability.
For the purpose of reducing the number of signal processing IC's and the number of steps of board mounting, there has been employed a structure utilizing matrix wirings. FIG. 1 shows a photoelectric conversion device utilizing such matrix wirings, wherein there are shown a photoelectric conversion device 1 consisting of a linear array of plural photoelectric conversion elements; a scanning unit 2; a signal processing unit 3; and matrix wirings 4. Among the wirings connecting the scanning unit 2 and the signal processing unit 3, vertical ones constitute individual electrodes while horizontal one constitute common lines.
In such a matrix wiring, the individual electrodes and common lines are inevitably close in order to compactize the matrix wiring. Consequently floating capacitances are present between the wirings, thus generating crosstalk among the output signals, and deteriorating the obtained image signals This drawback can be most simply resolved by increasing the distances between the wirings. However, such solution increases the dimension of the matrix wiring, and is not desirable for a device requiring a large number of photoelectric conversion elements as explained before.
There has also been proposed a photoelectric conversion device in which a conductor layer and a wiring capable of maintaining a constant potential at each crossing point of the individual electrode and the common line, thereby controlling the floating capacity between the individual electrodes and the common lines and preventing the crosstalk of the output signals released through such floating capacitances.
FIG. 2A is a plan view of matrix wiring in which a conductor layer of a constant potential is formed at the insulated crossing point of the individual electrode and the common line, while FIG. 2B is a cross-sectional view along a line B--B' of the matrix wiring shown in FIG. 2A. In FIGS. 2A and 2B there are shown individual electrodes 301-304; common lines 305-308; intermediate lines 309-313 positioned in the spaces of the common lines 305-308; a conductor layer 314 positioned between the individual electrodes 301-304 and the common lines 305-308 in the insulated crossing points thereof and is connected to an unrepresented power source capable of maintaining a constant potential; and contact holes 315 for ohmic contact of the individual electrodes 301-304 and the common lines 305-308.
However the photoelectric conversion device with such matrix wiring having a conductor layer of a constant potential in the insulated crossing points of the individual electrodes and the common lines have been associated with the following drawbacks.
In such matrix wiring, there is provided a conductor layer of a constant potential in the insulated crossing points of the individual electrodes and the common lines for reducing the capacitances therebetween.
Though such structure can reduce the floating capacitances between the individual electrodes and the common lines, new floating capacitances are formed between the conductor layer of constant potential and the individual electrodes and between said conductor layer and the common lines.
The floating capacitances are generated between the conductor layer of constant potential and all the individual electrodes and between said conductor layer and all the common lines because the conductor layer is extended over the entire area of the matrix wiring, and are practically not negligible.
FIG. 3 is an equivalent circuit of an accumulating photoelectric conversion device employing said conductor layer of constant potential. In this drawing, 501 denotes a photoelectric conversion unit. 503 denotes a storage capacitor. 502 denotes a switch unit. 505 denotes a load capacitor. 506 denotes a signal output terminal unit. If the above-explained matrix wiring is employed in the output side of the accumulating photoelectric conversion circuit shown in FIG. 3, a floating capacitance 504 not negligible in comparison with the load capacitor 505 which may deteriorate the efficiency of charge transfer.